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I've gotten to the point that I use my GPS speedo as much, if not more, than the one in my dash. But this could be a bad habit that causes me to get pulled over.

In principle the GPS speedo should be accurate for straight flat roads, but I wonder about the twisties in the hills or mountains. The 550 has a feature that tells you it is accurate within -- feet; it would be nice if it had a feature that tells you it is accurate within -- mph. Also my GPS is not routinely calibrated as are most peace officer speedos and radar units.

I've gotten to the point that I use my GPS speedo as much, if not more, than the one in my dash. But this could be a bad habit that causes me to get pulled over.

In principle the GPS speedo should be accurate for straight flat roads, but I wonder about the twisties in the hills or mountains. The 550 has a feature that tells you it is accurate within -- feet; it would be nice if it had a feature that tells you it is accurate within -- mph. Also my GPS is not routinely calibrated as are most peace officer speedos and radar units.

It's something to think about.

In actual fact, your GPS calibrates itself every time you turn it on and obtain new data blocks from the satellite constellation, or lock onto a new satellite rising over the horizon. They (satellites and your GPS) are timed by and synchronized to atomic clock references that are highly accurate which is necessary for accurate positioning. The civilian differential GPS units can be made accurate into inches of position if necessary, and military grade stuff is so accurate you can fly missiles into specific windows if you wish. CEP is essentially zero. The GPS speed readout is accurate maneuvering or in straight lines and is easily as accurate as speed radars and lasers for all practical uses. The more satellites you can see and are receiving at any given time, the better accuracy of position and speed calculation.

BTW, you would be surprised at how often, over 35 years, I have defended speeding ticket cases (no, not mine!) and find radars that have never been re-calibrated, properly licensed, or sometime operated by non-certified officers. I also have presented evidence from a logging GPS system to prove a speed radar was either defective or being improperly operated. The Judge chose to believe a twelve billion dollar satellite GPS system and vehicle logger rather than the $700 second-hand radar gun.

The civilian differential GPS units can be made accurate into inches of position if necessary, and military grade stuff is so accurate you can fly missiles into specific windows if you wish.

I don't think missiles don't fly under cover of trees. My motorcycle does and as a result I've had GPS units (Garmin) report speeds as high as 747 MPH while I was riding about 35 MPH. GPS units don't always handle multipath signals bounced off of tall city buildings very well, either. I've had a GPS report that I was doing in excess of 500 MPH... while stopped at a stop light. Those examples come from different GPS units.